Sn Anodes Protected by Intermetallic FeSn 2 Layers for Long-lifespan Sodium-ion Batteries with High Initial Coulombic Efficiency of 93.8.

With a theoretical capacity of 847 mAh g -1 , Sn has emerged as promising anode material for sodium-ion batteries (SIBs). However, enormous volume expansion and agglomeration of nano Sn lead to low Coulombic efficiency and poor cycling stability. Herein, an intermetallic FeSn 2 layer is designed via thermal reduction of polymer-Fe 2 O 3 coated hollow SnO 2 spheres to construct a yolk-shell structured Sn/FeSn 2 @C. The FeSn 2 layer can relieve internal stress, avoid the agglomeration of Sn to accelerate the Na + transport, and enable fast electronic conduction, which endows quick electrochemical dynamics and long-term stability. As a result, the Sn/FeSn 2 @C anode exhibits high initial Coulombic efficiency (ICE=93.8 %) and a high reversible capacity of 409 mAh g -1 at 1 A g -1 after 1500 cycles, corresponding to an 80 % capacity retention. In addition, NVP//Sn/FeSn 2 @C sodium-ion full cell shows outstanding cycle stability (capacity retaining rate of 89.7 % after 200 cycles at 1 C).